1. Field of the Invention
The present invention relates to an a.c. generator for a vehicle, and more particularly to the structure of a stator winding wound on a stator in an a.c. generator for a vehicle.
2. Description of the Related Art
FIG. 5 is a cross-sectional view showing a general a.c. generator for a vehicle.
Referring to FIG. 5, the a.c. generator for a vehicle is structured by rotatably mounting a Lundell-type rotor 7 inside a case 3 constructed from an aluminum front bracket 1 and an aluminum rear bracket 2 by means of a shaft 6, and fastening a stator 8 to an inner wall of the case 3 so as to cover an outer peripheral side of the rotor 7.
The shaft 6 is rotatably supported by the front bracket 1 and the rear bracket 2. A pulley 4 is fastened to one end of the shaft 6 so that the rotating torque of an engine can be transmitted to a shaft 6 by means of a belt (not shown). Slip rings 9 for supplying a current to the rotor 7 are fastened to the other end portion of the shaft 6, and a pair of brushes 10 are housed in a brush holder 11 disposed within the case 3 such that the pair of brushes 10 slide in contact with the slip rings 9. A regulator 18 for adjusting the magnitude of an a.c. voltage generated in the stator 8 is fastened by adhesive to a heat sink 17 fitted onto the brush holder 11. A rectifier 12 which is electrically connected to the stator 8 and rectifies the a.c. voltage generated in the stator 8 into a d.c. voltage is mounted within the case 3.
The rotor 7 is composed of a rotor winding 13 for generating magnetic flux on passage of electric current, and a pair of pole cores 20 and 21 disposed so as to cover the rotor winding 13, magnetic poles being formed in the pole cores 20 and 21 by magnetic flux generated in the rotor winding 13. The pair of pole cores 20 and 21 are made of iron, each has six claw-shaped magnetic poles 22 and 23 disposed on an outer circumferential perimeter at even pitch in a circumferential direction so as to project axially, and the pole cores 20 and 21 fastened to the shaft 6 facing each other such that the claw-shaped magnetic poles 22 and 23 intermeshed. In addition, fans 5 are fastened to both ends of the rotor 7 in the axial direction thereof.
The stator 8 is composed of a cylindrical stator core 15 having a plurality of slots extending axially at a predetermined pitch in a circumferential direction, and a stator winding 16 wound on the stator core 15.
In the a.c. generator for a vehicle thus structured, a current is supplied to the rotor winding 13 from a battery (not shown) through the brushes 10 and the slip rings 9, to thereby generate magnetic flux. The claw-shaped magnetic poles 22 of the pole core 20 are magnetized with north-seeking (N) poles by this magnetic flux, and the claw-shaped magnetic poles 23 of the pole core 21 are magnetized with south-seeking (S) poles. At the same time, rotating torque from the engine is transmitted through the belt and the pulley 4 to the shaft 6, rotating the rotor 7. Thus, a rotating magnetic field is applied to the stator winding 16, generating electromotive force in the stator winding 16. This a.c. electromotive force passes through the rectifier 12 and is converted into a d.c. electromotive force, the magnitude of the electromotive force is adjusted by the regulator 12 and the battery is recharged.
Now, the structure of the stator winding 16 will be described with reference to FIG. 6. In this example, the number of magnetic poles of the rotor 7 is 12. In FIG. 6, reference numerals 1 to 36 denote slot Nos.
The stator core 15 is formed of a cylindrical magnetic material in which 36 slots 15a are formed at a predetermined pitch in a circumferential direction so as to open toward the inner peripheral side thereof.
A winding 30 is formed by wave winding a conductor 29 into every three slots 15a such that the order of slots into which the conductor 29 is inserted is slot number 1, 4, 7, . . . ,34. A winding 31 is formed by wave winding a conductor 29 into every three slots 15a such that the order of slots into which the conductor 29 is inserted is slot number 3, 6, 9, . . . ,36. A winding 32 is formed by wave winding a conductor 29 into every three slots 15a such that the order of slots into which the conductor 29 is inserted is slot number 5, 8, 11, . . . ,2.
Those three windings 30, 31 and 32 thus formed is connected their winding end portions 30b, 31b and 32b together, that is, is connected into Y connection to structure the stator winding 16. The connecting portion of the winding end portions 30b, 31b and 32b becomes a neutral point of the stator winding 16, and winding start portions 30a, 31a and 32a become lead wires of the stator winding 16. Those three windings 30, 31 and 32 are different in electrical angle by 120xc2x0 phase.
The rectifier 12 is structured in such a manner that a plus side diode d11 and a minus side diode d21 which are connected in series, a plus side diode d12 and a minus side diode d22 which are connected in series, a plus side diode d13 and a minus side diode d23 which are connected in series, and a plus side diode d14 and a minus side diode d24 which are connected in series are connected in parallel between an output terminal B and the ground. Also, as shown in FIG. 7, the stator winding 16 is connected to the rectifier 12 in such a manner that the winding start portion 30a of the winding 30 is connected between the plus side diode d11 and the minus side diode d21 which are connected in series, the winding start portion 31a of the winding 31 is connected between the plus side diode d12 and the minus side diode d22 which are connected in series, the winding start portion 32a of the winding 32 is connected between the plus side diode d13 and the minus side diode d23 which are connected in series, and the neutral point is connected between the plus side diode d14 and the minus side diode d24 which are connected in series, respectively.
In the a.c. generator for a vehicle thus structured, if the rotating speed is low, a voltage of 3xc2xd times of a single-phase induced voltage is developed between the wires. Also, if the rotating speed is high, although main magnetic flux is weakened by a three-phase a.c. current that flows in three single-phase windings 30, 31 and 32, since a tertiary higher harmonic electromotive force remains, this generator is employed with the diode connected to the neutral point.
In the conventional a.c. generator for a vehicle, since the stator winding 16 is structured by connecting the three single-phase windings 30, 31 and 32 different in electrical angle by 120xc2x0 phase into Y connection, only the voltage of 3xc2xd times of the single-phase induced voltage is developed between the wires, resulting in such a problem that the conventional a.c. generator could not satisfy a demand for generation when the rotating speed is low.
In order to solve the above drawback, there is proposed that the number of turns of the windings 30, 31 and 32 in the respective phases is increased. However, in this case, there arises such a problem that a generation current decreases when the rotating speed is high.
The present invention has been made in order to solve the above problems with the conventional generators, and therefore an object of the present invention is to provide an a.c. generator for a vehicle with an improved stator winding which is capable of making a voltage developed between the wires large and making a generation current at a higher rotating speed large.
In order to achieve one aspect of the above object, according to one aspect of the present invention, there is provided an a.c. generator for a vehicle, including:
a rotor rotatably supported within a case; and
a stator having a cylindrical stator core in which a plurality of slots are formed at given intervals circumferentially so as to open toward the inner peripheral side thereof and a stator winding constructed by connecting three single-phase windings wound on the stator core into Y connection, the stator being fitted onto the case so as to surround the rotor,
wherein each of the single-phase windings includes a first winding which is connected into the Y connection and a second winding which is connected in series to the first winding and has a given phase difference in electrical angle with respect to the first winding, and
wherein a connecting portion of the first winding and the second winding constituting each of the single-phase windings is connected to a pair of three-phase diode bridges, and an end portion of the second winding constituting each of the single-phase windings is connected to another pair of three-phase diode bridges.